1. Field of the Invention
The invention relates to a disk drive apparatus and a spherical aberration correcting method which are suitable for use in an optical disk having a plurality of recording layers.
2. Description of the Related Arts
In recent years, as a recording medium for recording digital data, a recordable type optical disk is generally used. In the field of the optical disk, a plurality of kinds of recordable optical disks having different physical characteristics while keeping reproduction compatibility have been developed and put into practical use. Since those plurality of kinds of recordable optical disks have a recording capacity which is equivalent to that of a read only optical disk and also have the reproduction compatibility, they have rapidly been spread.
As such recordable optical disks, there are a CD-R (Compact Disc-Recordable) and a CD-RW (Compact Disc-ReWritable) as disks which conform with the standard of the CD (Compact Disc), and a DVD-R (DVD Recordable), a DVD+R (DVD +R format), a DVD-RW (DVD Re-Writable), a DVD+RW (DVD +RW format), and the like have been well-known as disks which conform with the standard of the DVD (Digital Versatile Disc). Since a recording capacity of the disk of the type which conforms with the standard of the DVD is large to be 4.7 GB (Giga bytes) or more, it is particularly remarkably spread.
In recent years, in association with a request to further increase the recording capacity in the market, the realization of the recordable type disks as products is progressing even with respect to the optical disk having a plurality of recording layers which has been used in the past only in the read only optical disk. A DVD-R double-layered disk is known as a recordable optical disk having a plurality of recording layers.
The recording and reproduction of data onto/from the optical disk are executed by irradiating a laser beam onto the recording layer of the optical disk from an optical pickup. That is, the laser beam is converged onto the recording layer and a light spot is formed on the recording layer. Upon recording, a recording mark is formed by altering a recording film by energy of the laser beam. Upon reproduction, the reflection light of the laser beam from the recording layer is detected by a photodetector and the recording mark (pit) is read. In the case of the DVD, a track pitch is set to about 0.74 μm and a diameter of the light spot which is formed on the recording layer is set to, for example, about 0.89 μm.
In the optical pickup, the laser beam is irradiated from a laser beam source, enters an objective lens through abeam splitter or the like, and is converged by the objective lens, thereby forming the light spot onto the recording layer of the optical disk. An image of the light spot is deformed by a spherical aberration which is caused by optical imperfection of the objective lens. For example, in the case of executing the recording and reproduction of the DVD, since it is necessary to form the light spot having the micro diameter as mentioned above onto the recording layer, it is desirable to correct the spherical aberration.
To correct the spherical aberration, there is generally used a method whereby, for example, a liquid crystal optical device on which concentric patterns have been formed is arranged on a position of the objective lens opposite to the disk surface side and transmittance of the liquid crystal device is concentrically controlled on the basis of a predetermined correction value. As a correction value, for example, it is possible to use a value obtained by measuring a reproduction jitter, an amplitude of a reproduction RF signal, a push-pull signal by a 2-split photodetector, or the like. In such a case, it is necessary to separately execute the recording/reproducing operation in order to obtain the correction value.
Since the laser beam enters a transmission layer (substrate), is refracted in a predetermined direction, and is irradiated to the recording layer, an adjustment value of the spherical aberration can be determined in accordance with a thickness of transmission layer. A method whereby the thickness of transmission layer is measured by a focusing search and used for correction of the spherical aberration has been proposed in JP-A-2003-91851.